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Ability to run multiple operating systems on one machine
Ability to use different file systems
A common source of confusion for users unfamiliar with Linux is the matter of how partitions are used and accessed by the Linux operating system. In DOS/Windows, it is relatively simple: Each partition gets a "drive letter." You then use the correct drive letter to refer to files and directories on its corresponding partition. This is entirely different from how Linux deals with partitions and, for that matter, with disk storage in general. This section describes the main principles of partition naming scheme and the way how partitions are accessed in {PRODUCT}.
A file system splits the remaining space into small, consistently-sized segments. For Linux, these segments are known as _blocks_ footnote:[Blocks really *are* consistently sized, unlike our illustrations. Keep in mind, also, that an average disk drive contains thousands of blocks. The picture is simplified for the purposes of this discussion.].
After creating a smaller partition for your existing operating system, you can reinstall software, restore your data, and start the installation. xref:figu-partitions-destructive-repartitioning[Disk Drive Being Destructively Repartitioned] shows this being done.
An extended partition is like a disk drive in its own right - it has its own partition table which points to one or more partitions (now called _logical partitions_, as opposed to the four _primary partitions_) contained entirely within the extended partition itself. xref:figu-partitions-extended[Disk Drive With Extended Partition], shows a disk drive with one primary partition and one extended partition containing two logical partitions (along with some unpartitioned free space).
An Introduction to Disk Partitions
An Unused Disk Drive
An unused hard disk also falls into this category. The only difference is that *all* the space is not part of any defined partition.
An unused partition is available
Any data previously present in the original partition is lost.
Aside from adding a new hard drive to your system, you have two choices:
A small percentage of the driver's available space is used to store file system-related data and can be considered as overhead.
As the following figure shows, the first step is to compress the data in your existing partition. The reason for doing this is to rearrange the data such that it maximizes the available free space at the "end" of the partition.
As the previous step implied, it may or may not be necessary to create new partitions. However, unless your resizing software is Linux-aware, it is likely that you must delete the partition that was created during the resizing process. xref:figu-partitions-final-configuration[Disk Drive with Final Partition Configuration], shows this being done.
As this figure implies, there is a difference between primary and logical partitions - there can only be four primary partitions, but there is no fixed limit to the number of logical partitions that can exist. However, due to the way in which partitions are accessed in Linux, no more than 12 logical partitions should be defined on a single disk drive.
As with most computer-related technologies, disk drives changed over time after their introduction. In particular, they got bigger. Not larger in physical size, but bigger in their capacity to store information. And, this additional capacity drove a fundamental change in the way disk drives were used.
As xref:figu-partitions-drive-with-data[Disk Drive with Data Written to It], shows, some of the previously-empty blocks are now holding data. However, by just looking at this picture, we cannot determine exactly how many files reside on this drive. There may only be one file or many, as all files use at least one block and some files use multiple blocks. Another important point to note is that the used blocks do not have to form a contiguous region; used and unused blocks may be interspersed. This is known as _fragmentation_. Fragmentation can play a part when attempting to resize an existing partition.
As xref:figu-partitions-formatted-drive[Disk Drive with a File System], implies, the order imposed by a file system involves some trade-offs:
As xref:figu-partitions-partition-table[Disk Drive with Partition Table] shows, the partition table is divided into four sections or four _primary_ partitions. A primary partition is a partition on a hard drive that can contain only one logical drive (or section). Each section can hold the information necessary to define a single partition, meaning that the partition table can define no more than four partitions.